Two studies show that a class of chemicals known as neonicotinoids created disorientation among bees and caused colonies to lose weight, which may have contributed to a mysterious die-off.

Scientists have identified a new suspect in the mysterious die-off of bees in recent years — a class of pesticides that appear to be lethal in indirect ways.

The chemicals, known as neonicotinoids, are designed to target a variety of sucking and chewing insects, including aphids and beetles. Bees are known to ingest the poison when they eat the pollen and nectar of treated plants, though in doses so tiny that it was not seen as a threat.

But two reports published online Thursday by the journal Science indicate that the pesticides are not altogether benign. One study found that bumblebee colonies exposed to amounts of the insecticide similar to what they'd encounter in the wild gained less total weight than colonies that weren't exposed. Another study used miniature radio frequency chips to track honeybees and found that the pesticide impaired their ability to navigate back to the hive after a feeding expedition.

"If it's blundering around and can't return to the hive ... the bee might as well be dead," said Christian Krupke, an entomologist at Purdue University in West Lafayette, Ind., who was not involved in either study.

Beekeepers became alarmed that honeybees were vanishing from their nests across the U.S. in the fall of 2006 — victims of a perplexing and pervasive malady now known as colony collapse disorder that wiped out as many as 90% of bees, in some cases. Scientists don't know exactly why the ailment strikes, but they believe it results from a combination of habitat degradation, infection by pathogens and parasites and pesticide use. Researchers have also documented sharp declines in bumblebees, which are important crop pollinators but are not domesticated.

The fate of bees is important because they pollinate so many plants, said David Goulson, a biologist at the University of Stirling in Scotland who led the study involving bumblebees. Losing too many of the world's bees could endanger wildflowers, which in turn would affect birds and butterflies, he said.

Bees also help pollinate scores of fruit and vegetable crops, a service that's estimated to be worth $15 billion to the agricultural industry. In California alone, growers rely on honeybees to pollinate 90 crops, including avocado, almond, cherry and plum trees as well as vegetables grown from seeds. In the absence of plentiful bees, California growers could be forced to spend about $250 million renting bees to fertilize their crops.

Neonicotinoid pesticides were developed to eradicate insects without threatening mammals. The chemicals, which are incorporated into the tissues, leaves and flowers of plants, target the central nervous system, leading to paralysis and death. Farmers began using them in the early 1990s.

Past studies have explored effects of neonicotinoids in the lab, finding that they might harm bees' memory, learning and orientation. But the new studies are among the first to examine the pesticides' effects on bees under real-world conditions.

In Scotland, Goulson and his colleagues brought colonies of bumblebees indoors, feeding some a diet of pollen and nectar tainted with neonicotinoids and giving others food without the chemicals. Then they let both groups of bees out in an enclosed field site and let them forage for six weeks.

By the end of the study period, the bees that ate the insecticide produced 85% fewer queens per colony, on average. The queens lay the eggs that produce the members of the colony for the next year.

"If that went on for years, the consequences could be pretty dramatic," Goulson said.

The colonies that were exposed to the pesticide also gained 8% to 12% less weight — including honey and beeswax — than the control colonies.

The second study, led by researchers from the French National Institute for Agricultural Research, or INRA, focused on honeybees, which have been victimized by colony collapse disorder throughout the Northern Hemisphere.

First they glued special radio frequency identification tags to the bees' thoraxes. Then they fed the bees sublethal doses of a neonicotinoid and monitored the insects as they attempted to return to the hive.

The research team discovered that the "intoxicated" bees were about twice as likely as unexposed bees to die because they couldn't find their way home. Computer simulations suggested that these no-shows could cause hive populations to crash in a matter of weeks, said study coauthor Mickael Henry, a researcher at INRA in Avignon.

The weakened colonies would be especially vulnerable to environmental stresses such as climate change or disease, he added.

The new findings lend support to the notion that pesticides contribute to colony collapse but leave open the likelihood that habitat destruction and illness play a role too, scientists said.

"There are a whole lot of things that stress the honeybees," said Eric Mussen, a honeybee specialist at UC Davis. "You can't point your finger at one thing and say, 'That is the problem.' "

Mussen cautioned against singling out neonicotinoids when other pesticides could have similar effects on bees. Besides, he said, many insects have built up immunity to neonicotinoids, so farmers are likely to switch to different pesticides anyway.

Jeff Pettis of the Department of Agriculture's bee research lab in Beltsville, Md., who wasn't involved in the studies, praised the bumblebee report in particular for highlighting that honeybees aren't the only ones that may suffer from sublethal doses of pesticides. He predicted that the effects on bee reproduction would raise red flags for regulators at the Environmental Protection Agency.

"There's a general phenomenon of pollinator decline — bats, bird, butterflies, all kinds of things," he said.